In Wideband Code Division Multiple Access (WCDMA) networks and Evolved Universal Terrestrial Radio Access Networks (E-UTRAN) the mobility decisions, which include cell reselection and handover heavily rely on cell search of new cells and subsequently the downlink measurements of these identified cells performed by the UE. Therefore, in order to ensure good UE mobility performance the UE should be able to measure and keep track of certain number of best cells in terms of downlink measured quality.
In WCDMA in active mode the UE is required to detect (or identify) a new intra-frequency cell in at least 800 ms. In addition it should be able to measure and report the downlink quality (i.e. CPICH measurements) of at least 8 cells (one serving and seven neighbor cells), while fulfilling the minimum performance requirements as specified in TS 25.133[1].
In E-UTRAN the measurement performance requirements along the lines similar to those used in WCDMA are likely to be specified [2].
Mobility Scenarios
There are basically two kinds of mobility scenarios:
Idle mode mobility: cell reselection
Connected mode mobility: handover
The cell reselection is mainly UE autonomous function without the intervention of its serving cell. But to some extent the UE behavior in this mobility scenario could still be controlled by some broadcasted system parameters and performance specification.
The handover on the other hand is fully controlled by the network through explicit UE specific commands and by performance specification.
In both idle and connected modes the mobility decisions are mainly based on the same kind of downlink measurements as will be discussed in more details in relation to the description of the problem.
Both WCDMA and E-UTRAN are frequency reuse-1 systems. This means the geographically closest neighbor cells operate on the same carrier frequency. An operator may also deploy multiple frequency layers within the same coverage area. Therefore, idle mode and connected mode mobility in both WCDMA and E-UTRAN could be broadly classified into three main categories:
Intra-frequency mobility (idle and connected modes) where the UE moves between the cells belonging to the same carrier frequency. This is the most important mobility scenario since it involves less cost in terms of delay due. In addition an operator would have at least one carrier at its disposal that it would like it to be efficiently utilized. Inter-frequency mobility (idle and connected modes) is the scenario where the UE moves between cells belonging to different carrier frequencies but of the same access technology. This could be considered as the second most important scenario. Inter-RAT mobility (idle and connected modes) is the scenario where the UE moves between cells that belong to different access technologies such as between WCDMA and GSM or vice versa.
UE Measurement Aspects Related to Mobility
In order to guarantee good mobility performance in WCDMA or in E-UTRAN UE performs two major tasks:
Identification of new cells, i.e. to fully synchronize and identify an unknown cell Regularly measure and report the downlink measurements on some plot or reference signal channels from certain minimum number of identified cells, e.g. CPICH Ec/No and RSCP in WCDMA [3] and RSRP and RSRQ in E-UTRAN.
In order to ensure good mobility performance the requirements related to the above measurement tasks are specified. In WCDMA the specification covers the following is specified to ensure that UE meets at least these requirements. Some of these important requirements in active mode include [1]:
Identification delay of unknown cells for the given received level of the corresponding synchronization and CPICH signals. The max delay is up to 800 ms for intra-frequency cells in continuous reception mode (i.e. without DRX).
Minimum number of identified cells (8 including one serving and 7 neighbor cells) for which UE is supposed to report the CPICH measurements with the specified measurement absolute and relative accuracies
The measurement period of 200 ms over which the specified measurement accuracies of at least 8 cells are fulfilled
Similar requirements will be specified for E-UTRAN.
Cell Identification Process in UE
In order to report the downlink measurements of the requirement number of identified cells to the network, the UE needs to regularly look (i.e. identify) for new cells that have become better than the current or old cells. The terms cell detection, cell search and cell identification have the same meaning, which is the UE ability to fully synchronize (i.e. finding cell timing, cell identity etc) to previously unknown cell. The cell to be identified may or may not belong to the neighbor cell list provided by the network to the UE.
Thus to fulfill the standardized minimum requirements and due to the varying radio conditions the UE needs to identify and measure the neighbor cells on regular basis. The scheduling of such measurement process is not standardized but is rather UE implementation specific. To limit hardware cost and to prevent battery exhaustion the UE would typically collect measurement samples for different type of measurements at some periodic intervals.
In idle mode the UE does measurement mainly at the paging occasions (i.e. at the wake instances at the end of DRX cycle). Therefore, measurement sampling rate in idle mode is considerably low compared to the connected mode scenario. Due to this reason measurement performance in idle mode becomes much coarser than that would be achievable in connected mode.
Impact of Discontinuous Reception (DRX) in Active Mode on Cell Identification
In WCDMA release 7 a new feature called discontinuous reception (DRX) in connected mode (more specifically in CELL_DCH state) has been specified [5]. This allows UE to save its battery while stay connected since it wakes up only at periodic instances according to the DRX cycle. In release 7 the maximum DRX cycle is 40 ms.
In general the DRX feature also implies that UE will mainly collect the measurement samples at the wake up instances or at least less frequently compared to the continuous reception case. Accordingly the measurement requirements have been relaxed in DRX mode in release 7 [1]. The worst case intra-frequency cell identification in DRX (release 7) may take up to 6 seconds, i.e. if we assume 40 ms DRX and 5% UE activity level [1]. Therefore, DRX in active mode may have some adverse effect on the mobility performance in general and cell identification performance in particular. However, the delay of 6 seconds is still within an acceptable limit for packet data services, which are more delay tolerant. It has been shown that this will affect only up to 10% of the users. This is because in most cases the active set update, where one or more old cells are replaced by the newly identified ones, takes longer than 6 seconds. However, for real time services, even better performance is desirable.
Introduction of new services and future enhancements in WCDMA will likely require more DRX options. However, the current requirements, which are based on state of the art principles, would be inadequate to meet the mobility performance target (i.e. reduce call dropping rate).
In E-UTRAN the active mode DRX may allow network to use up to 2.56 seconds of DRX cycle. Thus there is risk that if current methodology is used the cell identification may also become unnecessarily longer.
Problems with Existing Solutions
Cell Identification in DRX Scenario
In DRX mode if UE identifies a new cell only during the active times then obviously the delay will be longer especially if the target cell is weak in terms of its reception quality, e.g. CPICH Ec/No and/or SCH Ec/No.
On the other hand if all the time in DRX mode the UE is forced to provide the same cell identification performance as achieved in continuous reception mode would drain UE battery. Thus there will be no use of having a DRX feature.
Poor measurement performance and especially longer cell identification delay would delay the active set update. This in turn would also delay handover decisions at the base station, which relies on UE measurement reports for executing handover. Therefore, in active mode the performance degradation of these measurements should be minimized to prevent unnecessary call dropping.
Several state of the art solutions exist or are proposed so as to minimize the performance degradation of cell identification in active mode DRX scenario, of which a few is presented in the following:
Identify Cells with Higher SCH and CPICH Received Levels
The currently agreed solution is that UE in DRX performs measurement and identify cells provided that the minimum received level on synchronization channel (SCH) and common pilot channel (CPICH) on that cell is significantly higher (e.g. 3 dB higher than in case of non DRX). Since cells are relatively stronger therefore on the average the cell identification will be relatively faster.
Absolute Thresholds Based Cell Identification Delay Adjustment
The network provides UE with absolute thresholds in terms of CPICH Ec/lo or CPICH RSCP levels and some measurement activity factor. The latter parameter is used to scale the measurement activity (e.g. cell identification time, measurement period) depending upon the CPICH Ec/lo and/or CPICH RSCP reception level of the strongest monitored cell in the active set.
As long as the received CPICH Ec/No and/or CPICH RSCP from the serving cell are above these signalled thresholds, the UE has to identify a new intra-frequency cell according to the measurement activity factor. Generally this would lead to longer cell identification delay depending upon the signaled activity factor. On the other hand when this condition is no longer met, i.e. if the serving cell quality is weak (below Ec/No or/and RSCP thresholds) then the UE will identify a new cell with the same delay as specified for continuous reception case, i.e. 800 ms worst case delay [1].
One drawback is that UE will have to measure more often even if it is not needed for instance there may still be relatively good cells in the active set. Thus in reality UE will have to wake up during the inactive time of the DRX length. This will drain UE battery thereby defying the benefits of DRX operation.
Switch to Continuous Mode (Non DRX) in Cell Border Region
The WCDMA specification [5] allows network in active mode to promptly direct the UE to switch between DRX and non DRX modes through low level signaling. Thus, one possible solution to speed up the measurement process is to revert to non DRX mode (or very short DRX cycle) when UE enters in cell border region. The network can determine whether UE lies in cell border region or not by comparing the UE reported downlink channel quality (e.g. CPICH level) with a certain threshold. Typically between 25-35% users operate in the cell border region. Thus with approach on the average between 65-75% of the time the UE could barely stay in DRX. Since UE cannot fully utilize DRX, this approach is not desirable from UE battery saving perspective.